1. Field of the Invention
The present invention relates to optical devices, and more particularly, to optical devices such as optical interleavers and polarization-dependent optical isolators used for WDM (wavelength division multiplexing) optical communication.
2. Related Background Art
FIG. 21 shows an example of a conventional optical demultiplexer 900 made of a natural birefringent material, such as rutile. The demultiplexer 900 includes an ingoing optical fiber 901, an ingoing lens 902, a birefringent material 903 such as rutile, a first outgoing lens 904, a first outgoing optical fiber 905, a second outgoing lens 906, and a second outgoing optical fiber 907. Incident light that is coupled from the ingoing optical fiber 901 through the ingoing lens 902 into the birefringent material 903 is separated into ordinary light (TE wave) 909 and extraordinary light (TM wave) 908. The ordinary light 909 and the extraordinary light 908, which are separated by a distance proportional to the length of the birefringent material 903, are then coupled by the outgoing side lenses into the first outgoing optical fiber 905 and the second outgoing fiber 907.
Natural birefringent materials, such as rutile, have a polarization-dispersion surface (refractive index ellipsoid) as shown in FIG. 22. For this reason, the light that is incident on the birefringent material 903 propagates in directions perpendicular to the dispersion surface of the ordinary light and to the dispersion surface of the extraordinary light, in accordance with the law of conservation of momentum.
However, the difference between the dispersion surface of ordinary light and the dispersion surface of extraordinary light for natural birefringent materials such as rutile is small, and consequently, the separation angle of ordinary and extraordinary light is small as well. Therefore, the length of the birefringent material has to be long, so that the demultiplexer 900 becomes large.
On the other hand, optical devices using photonic crystals in which the refractive index changes periodically have been proposed (see JP 2000-180789A, JP 2000-241762A, JP 2000-241763A and JP 2000-284225A). It should be noted that throughout this specification, xe2x80x9cphotonic crystalxe2x80x9d means an artificial multi-dimensionally periodic structure having a periodicity of the same order as the wavelength of light.
Conventional optical isolators that use photonic crystals as polarizers use photonic crystals having a photonic band structure in which either TE waves or TM waves are reflected at the ingoing plane. Therefore, the photonic crystal is tilted from the optical axis such that the reflected light (returning light) from the photonic crystal is not coupled into the light-source side. Such an optical isolator reflects light that is incident light from the light-source side, so that a separate optical system or optical design is necessary to make sure that the reflected light is not coupled into the light-source side, which makes the structure more complicated.
In view of these problems, it is an object of the present invention to provide a small optical device, with which incident light can be demultiplexed.
In order to achieve the object of the present invention, an optical device (CL1) according to the present invention includes a first optical member separating an incident light of wavelength xcex into TE wave and TM wave; and an optical input portion, which inputs the incident light into the first optical member; wherein the first optical member has a periodically changing refractive index; wherein an angle defined by a first reciprocal lattice vector xcex11 and a second reciprocal lattice vector xcex12 of the first optical member at the wavelength xcex is not larger than 90xc2x0; wherein, in the direction of the first reciprocal lattice vector xcex11, the wave number of the TE wave is larger than the wave number of the TM wave; wherein, in the direction of the second reciprocal lattice vector xcex12, the wave number of the TE wave is smaller than the wave number of the TM wave; and wherein the optical input portion inputs the incident light in a direction that is parallel to a plane P12 including the first reciprocal lattice vector xcex11 and the second reciprocal lattice vector xcex12. With this optical device, the difference between the dispersion surfaces of the TE waves and the TM waves in the demultiplexing portion becomes large, and the separation angle of the TE waves and the TM waves can be set to be large. As a result, it is possible to separate TE waves and TM waves at a propagation distance at which the influence of diffraction can be ignored, and it is possible to reduce the number of optical parts, such as lenses, and to make the device smaller.
Furthermore, another optical device (CL6) of the present invention includes, in addition to this optical device (CL1), a phase retarder and an optical output portion; wherein the first optical member, the phase retarder and the optical output portion are arranged such that light that enters from the optical input portion is transmitted in that order; wherein the optical input portion inputs a plurality of p light beams (wherein p is an integer), whose wavelengths range from a wavelength xcex(1) equal to xcex and increase at constant wavelength intervals to a wavelength xcex(p), in a direction that is parallel to the plane P12; and wherein the phase retarder imparts a difference in polarization between light beams of odd-numbered wavelengths and light beams of even-numbered wavelengths. With this optical device, the first optical member and a phase retarder imparting a constant polarization difference between odd-numbered wavelengths and even-numbered wavelengths are integrated into one optical device. Therefore, combining two kinds of polarizations and two kinds of wavelengths (odd-numbered wavelengths and even-numbered wavelengths), this optical device can be applied for devices with a variety of functions. This optical device is particularly preferable for WDM optical communication with light of a plurality of wavelengths that are separated by a constant wavelength interval.